Abstract
Nanocrystallinity is an important phenomenon in natural and synthetic systems. Whether an inorganic or organic material, the mechanism for confinement of crystal growth within nanoscopic dimensions usually employs an organic mediator, which adsorbs to and stabilizes the growing crystal. Following an introduction to nanocrystalline biomaterials and inorganic materials, this review focuses upon the preparation and study of nanoconfined crystalline organic polymers. Confinement is limited to nanoscopic domains, by the phase segregation of crystalline-amorphous block copolymers in the bulk or in solution. The effects of nanoconfinement upon the crystallization behaviors are discussed, including reports of small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) studies. Examples of nanoconfinement vs. break-out crystal growth are presented. Finally, irreversible trapping of nanocrystalline domains via covalent crosslinking of core-crystallized polymer micelles in solution is highlighted as a method for the preparation of permanent, discrete nanocrystalline objects. Confinement within the nanoscopic core domain of the shell crosslinked knedel-like (SCK) polymer micelle and covalent attachment to the crosslinked SCK shell impart interesting perturbations in local chain packing and disrupt the extent of crystallinity. In addition, SCKs with a toroidal morphology are prepared by micellization of diblock copolymers at high concentrations. A variety of physicochemical techniques are employed to characterize the thermal properties for SCKs of differing size and morphology. DSC is used to monitor the melting and crystallization transitions and to assess the percentage of the crystallinity of poly(η-caprolactone) (PCL) within the SCK cores, quenched at different temperatures from the melt. The core-shell morphology of the SCK imposes a fractionated crystallization onto the PCL and induces homogenous and heterogeneous nucleation. In addition, the low percentage of crystallinity found for PCL in SCKs, relative to that for PCL homopolymer and PCL-b-PAA copolymer is interpreted as the product of spatial confinement and covalent attachment to a crosslinked surface (the shell) provided by the containment of the PCL chains within the SCK core domain.
Keywords: nanocrystalline, biomacromolecules, differential scanning calorimetry (dsc), crystallization, dendritic hybrid, size-exclusion chromatography (sec), glassy polystyrene
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Catalytic C-H bond activation as a tool for functionalization of heterocycles
The major topic is the functionalization of heterocycles through catalyzed C-H bond activation. The strategies based on C-H activation not only provide straightforward formation of C-C or C-X bonds but, more importantly, allow for the avoidance of pre-functionalization of one or two of the cross-coupling partners. The beneficial impact of ...read more
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